It is known that dissolved substances (e.g. salts) can be separated from their solvents (e.g. water) by a technique known as "reverse osmosis". For example, potable or less salty water can be obtained from seawater, contaminated water, brackish water, or brine by this technique. Similarly, a "soft" water or relatively deionized water or water with reduced "total dissolved solids" can be obtained from a relatively "hard" water by the same or a similar technique. The latter application of the technique can be important in industry, e.g. for providing a relatively "soft" (calcium- and magnesium-free) water from a relatively "hard" tap water.
The use of this technology which has probably gained the widest attention to date is the desalination of brackish water or seawater to provide large volumes of relatively non-salty water for industrial, agricultural, or home use. What is involved here is literally a filtering out of dissolved ions or molecules by applying pressure to the seawater or other water solution and forcing the water through the reverse osmosis membrane, so that purified water passes through the membrane and the salt is "rejected" (i.e. filtered out). Osmotic pressure works against the reverse osmosis process, and the more concentrated the feed water, the greater the osmotic pressure which must be overcome.
To be practical, a reverse osmosis membrane must have very high "salt rejection" characteristics. If the concentration of the salt or other solute is not reduced by at least a factor of 10 in the first pass through the membrane, the solute level in the effluent water will still be too high for most purposes. Indeed, many commercial applications of the permeate or purified water required that the solute concentration be reduced by a factor of approximately 50 or more.
Reverse osmosis membranes have been prepared from a wide variety of known or preformed polymeric materials. Many of these known reverse osmosis membranes can reduce the concentration of solute by a factor of more than 50 (i.e. they have "salt rejection" capabilities in excess of 98%). However, a high salt rejection capability is not by itself enough for commercial practicality. In addition, the membrane should permit high flow rates or fluxes at reasonable pressures. For example, if in the case of seawater an applied pressure of 100 atmospheres provided a flux rate of less than ten gallons/ft.sup.2 -day (ten gfd, equivalent to about 410 liters/m.sup.2 -day), the volume of water processed by the membrane per unit of membrane surface would be insufficient for most of the important applications of the technology. Currently, process economics indicate a need for membrane fluxes of 600 to 800 liters per m.sup.2 -day (l/m.sup.2 -day) at pressures of 55 to 70 atmospheres for seawater feed (35,000 to 42,000 parts per million total dissolved salts). For brackish waters containing 3,000 to 10,000 parts per million (ppm) salts, economically attractive membranes preferably provide permeate fluxes of 600 to 800 l/m.sup.2 -day at pressures of only 25 to 40 atmospheres. While specific reverse osmosis applications for permselective membranes may deviate from these requirements, such membranes will not ordinarily achieve broad commercial applicability unless they meet these criteria.
In typical commercial applications of reverse osmosis technology, several additional criteria may be imposed by the realities of such applications. Among such additional requirements or criteria are high durability and resistance to compression, resistance to degradation by extremes of pH or temperature, resistance to microbial attack, and stability toward potentially corrosive or oxidative constituents in the feed water. A common constituent in some types of feed water is some form of chlorine which can oxidatively attack some of the prior art membranes. According to U.S. Pat. No. 3,951,815 (Wrasidlo), issued Apr. 20, 1976, the site of attack by chlorine on polyamide membranes is the amidic hydrogen present in the --CO--NH-- group. In compositions such as the polypiperazine-amides described in U.S. Pat. Nos. 3,687,842 (Credali et al), issued Aug. 29, 1972, 3,696,031 (Credali et al), issued Oct. 3, 1972, and the 3,951,815 patent cited previously, resistance to oxidative chlorine-containing species (e.g. hydrochlorite) in feed waters appears to have been adequately demonstrated. However, such resistance to attack by chlorine is believed to be atypical for polyamides in general.
In the manufacture or preparation of reverse osmosis membranes, variations are possible, not only with respect to the chemistry of the raw materials, but also with respect to polymerization techniques, lamination techniques, and the like. It presently appears that the optimum type of reverse osmosis membrane is extremely thin, to allow for high fluxes, but as free of flaws or imperfections as polymer chemistry and processing will permit. The two goals of minimal thickness and freedom from flaws are not altogether compatible. As the thickness of any polymeric film or membrane gets below five micrometers (.mu.M) and approaches molecular thickness (molecular thickness would be on the order of a few nanometers or even less), the probability of holes in the membrane or film structure increases significantly. A given area of membrane surface flawed by even a minimal number of holes of larger-than-molecular size could result in orders of magnitude losses in ppm of solute rejection. Accordingly, much of the technique in this art has been directed toward making extremely thin membranes which are essentially hole-free. Such extremely thin films or membranes lack structural integrity, in the free standing state, whether in the form of sheets or hollow fibers. The integrity can be improved by casting a solution of the polymer in the form of thick, asymmetric films or fibers in which a thin dense surface layer is supported by a porous spongy underlayer of the same material. Another approach involves casting the film or membrane onto a porous support. The porous support can be relatively thick, since it contains a great multitude of holes of larger-than-molecular size, and the structural integrity contributed by such a support does not necessarily reduce the flux.
If the polymer is to be cast from solution, it is normally essential that the polymer have a reasonable level of solubility in some suitable solvent; such solvent-soluble polymers are typically linear and can be assumed to have a crosslink density at or near zero (e.g. less than one crosslink per 100,000 molecular weight).
On the other hand, if the polymeric film or membrane is formed in situ on the support surface, e.g. through chain extension and/or crosslinking of monomes and/or prepolymers, solubility of the ultimate product (i.e. the thin film or membrane is not essential. In situ polymerization has been used to form a desalination membrane on the surface of a porous support. The membranes thus formed can be far thinner than five micrometers, although thicknesses below 10 nanometers are difficult to achieve in practice, typical thicknesses ranging from 10 or 20 to 1,000 nanometers.
The experience of at least one investigator, P. W. Morgan seems to indicate that interfacially-formed polyamide films prepared directly from the monomers tend to have too much permeability for reverse osmosis, except in those cases where the solute molecules are relatively large. According to Morgan's Condensation Polymers, Interscience Publishers, 1965, page 51, in-situ polymerized polyamides formed directly from the monomers can be used in osmosis experiments, but in the washed, undried state, "6-10 polyamide films were readily permeable to inorganic salts and to small dye molecules". Accordingly, although the sweep of polyamide chemistry is extremely broad and highly developed, it would appear from experiences such as those of Morgan that only a portion of this broad sweep can be brought to bear on the problems of reverse osmosis technology. And, as noted previously, polyamides formed from polycarboxylic acids or their functional equivalents and primary polyamines (i.e. polyamides having amidic hydrogen) can be sensitive to attack by agents such as the hypochlorites. Still another limitation on the use of polyamide chemistry is suggested in Richter et al, U.S. Pat. No. 3,567,632, issued Mar. 2, 1971. This patent discloses reverse osmosis desalinationmembranes formed from essentially linear aromatic polyamides, which polyamides have a high solubility in one or more selected solvents. Richter et al point out that the solubility requirements appears to be a critical restriction on reverse osmotic desalination performance.
The art of polyamide chemistry, even as applied to the specific field of reverse osmosis membranes has become so vast in recent years that it is difficult to provide an exhaustive list of pertinent citations from the patent and scientific literature. The following citations are believed to be representative.
I. U.S. Patents PA0 II. Scientific Literature
______________________________________ Patent No. Patentee Issue Date ______________________________________ 3,260,691 Lavin et al July 12, 1966 3,367,504 Westmoreland February 6, 1968 3,417,870 Bray December 24, 1968 3,480,588 Lavin et al November 25, 1969 3,551,244 Forester et al December 29, 1970 3,567,632 Richter et al March 2, 1971 3,597,393 Bach et al August 3, 1971 3,600,350 Kwolek August 17, 1971 3,615,024 Michaels October 26, 1971 3,619,424 Blanchard November 9, 1971 3,642,707 Frazer February 15, 1972 3,648,845 Riley March 14, 1972 3,663,510 Peterson May 16, 1972 3,687,842 Credali et al August 29, 1972 3,690,811 Horning September 12, 1972 3,692,740 Suzuki et al September 19, 1972 3,696,031 Credali et al October 3, 1972 3,710,945 Dismore January 16, 1973 3,744,642 Scala et al July 10, 1973 3,878,109 Ikeda et al April 15, 1975 3,904,519 Mckinney et al September 9, 1975 3,920,612 Stephens November 18, 1975 3,926,798 Cadotte December 16, 1975 3,951,815 Wrasidlo April 20, 1976 3,993,625 Kurihara et al November 23, 1976 3,996,318 van Heuven December 7, 1976 4,005,012 Wrasidlo January 25, 1977 4,020,142 Davis et al April 26, 1977 4,039,440 Cadotte August 2, 1977 4,048,144 Stephens September 13, 1977 4,051,300 Klein et al September 27, 1977 ______________________________________
Condensation Polymers, Chapter II, Interscience Publishers, 1965. PA1 S. Sourirajan, Reverse Osmosis and Synthetic Membranes, National Research Counsil of Canada, 1977, Chapter 9 (by P. Blais). PA1 NTIS Report No. PB 253 193/7GA (April, 1976). PA1 Office of Saline Water Research and Development Progress Report No. 359, October, 1968.
Of the foregoing patents, U.S. Pat. Nos. 3,567,632, 3,600,350, 3,710,945, 3,878,109, 3,904,519, 3,920,612, 3,951,815, 3,993,625, and 4,048,144 contain typical disclosures illustrating the formation of permselective membranes from polyamides (including aromatic polyamides) or their polyamine and polycarboxylic starting materials or precursors or chain-extenders. Also of interest in this regard are U.S. Pat. Nos. 3,619,424, 3,996,318, 4,005,012, 4,020,142, and 4,039,440. U.S. Pat. Nos. 3,260,691 and 3,480,588 relate to coating compositions prepared from condensation products of aromatic primary diamines and aromatic tricarboxylic compounds. U.S. Pat. Nos. 3,744,642 and 3,996,318 contain extensive discussions regarding the technique of interfacial polycondensation or other polymerization reactions conducted at an interface, particularly with respect to the utility of this technique in making reverse osmosis or desalination membranes. Several of the foregoing references include descriptions of membrane shapes or membrane composites designed primarily for the practice of reverse osmosis, purification of a water feedstock, or other permselective processes. Among these are U.S. Pat. Nos. 3,367,504, 3,417,870, 3,648,845, 3,926,798, 4,039,440, and 4,051,300. The preparation and properties of polysulfone support films is described in the Office of Saline Water Research and Development Progress Report No. 359 of October, 1968. Chapter 9 of the book Reverse Osmosis and Synthetic Membranescontains an extensive list of polyamide membranes and includes a discussion of their fabrication and properties. The polyamide disclosed in this reference are additionally described in several of the previously cited patents, including U.S. Pat. Nos. 3,567,632, 3,600,350, 3,687,842, 3,696,031, 3,878,109, 3,904,519, and 3,993,625. See also the previously cited NTIS Report of April, 1976.